Modern hydraulic brake systems for vehicles employ two independent hydraulic circuits fed by a double chamber master cylinder. This reduces the likelihood of a total loss of braking efficiency due to a single leak or rupture in the hydraulic system. Two ways of dividing the braking system into these two circuits are currently employed. In the "vertical" system the brakes for the two front wheels are joined in one circuit and the two rear wheels' brakes are joined into the second circuit. In the "diagonal" system the right front and left rear brakes are paired in one circuit and the left front and right rear brakes are paired in the other.
Various antiskid control devices have been developed to prevent the wheels of a vehicle from locking or skidding during braking. In general terms these devices function by controllably moderating the amount of braking action by moderating brake fluid pressure at the brakes at each wheel. This moderating can take place on each of the four wheels individually or it can be arranged to act on individual brake circuits and the two wheels of each such circuit involved.
An antiskid control device generally consists of a wheel speed detecting means, a computational arithmetic means capable of calculating the slip ratio based on wheel speed and an acceleration and a control means to control (modulate) wheel brake force based on the output signal from the computational arithmetic means by modulating hydraulic pressure with electronically controlled valves. Such devices are capable of decelerating a vehicle while regulating the wheel deceleration speed within an appropriate slip ratio by controlling the braking pressure according to the slip conditions of the wheel.
Known antiskid control methods include a diagonal type which controls and modulates the pressure applied to the brake circuit driving the brakes at one front wheel and the opposite rear wheel as pairs and vertical type which controls and modulates the pressure applied to the brakes at the two front wheels and the two rear wheels as pairs.
Conventional antiskid control devices have a number of disadvantages:
(1) Although substantially ideal antiskid control can be realized by providing a braking pressure control device and circuits which control the pressure applied to the brakes at each of the four wheels individually, additional regulating circuits, safety monitoring circuits and other various components are required in addition to the above-mentioned braking pressure control device and circuit for each wheel. This results in high costs and necessitates a large accommodating space. PA1 (2) It is structurally impossible for either the diagonal or vertical type brake control device to independently control the fluid pressure for each wheel in that each of the wheels is paired with another wheel. This creates problems under certain road conditions which result in the application of different coefficients of friction. Under such conditions, the braking pressure for each pair of wheels (whether paired with one another vertically or diagonally) is controlled according to the behavior of the wheel subjected to the lower coefficient of friction. This results in a longer stopping distance because the braking force of the wheel having the higher coefficient of friction is insufficient, (i.e., the braking pressure could be higher and still not result in skidding. PA1 (3) Japanese Application Laid-Open Print No. 59-206248 discloses a diagonal type brake system. This system makes it possible to simultaneously control the braking pressure pressures of both wheels which are connected to a pair of pressure-mediating circuits. However, such a system results in inferior performance as compared with a brake system having three or four pairs of control lines. PA1 (4) European Patent No. 51801 discloses a diagonal type dual brake circuit comprised of a decreased number of control lines, a single-type pressure modulator to provide for a pressure mediating circuit, and a so-called Select-High/Select-Low principle is used to temporarily determine and control the amount of pressure on both circuits in order to favorably regulate the braking pressure under varying circumstances. However, this system does not provide for the desired degree of stability and steerability because both stability and steerability deteriorate as the stopping distance is decreased.